Vacuum tube heater



y 4, 1954 F. E. GEHRKE ETAL 2,677,732

VACUUM TUBE HEATER Filed Oct. 27, 1950 INVENTORS FORREST E. GEHRKEWILLIAM T. GRANGER Patented May 4, 1954 VACUUM TUBE HEATER Forrest E.Gehrke, Huntington, and William T.

Granger, Floral Park, N. Y., assignors to Vania Electric ProductsMassachusetts Syl- Inc., a corporation of Application October 27, 1950,Serial No. 192,442

'7 Claims. 1

This invention relates to tubes.

In the manufacture of heaters for vacuum tubes, it has been known towind the heater wire in the form of a coil around a, heavier mandrelwire, the latter being first coated with insulation to avoidshort-circuiting the heater turns. Difficulties have been experiencedwith such heaters, these commonly being burnouts of the heater duringexhausting and aging of the tubes.

While we do not wish to be bound to our theory as to the probable causesof these burnouts, it was concluded that they may be due to thedevelop-- ment of pressure between the heater and mandrel due todifferences in expansion of the heater and mandrel respectively, and/ordue to the migration during processing and operation of the tube, ofmetal into the body of insulator between the heater and mandrel rod toan extent sufiicient to cause the formation of a conductive path betweenthe heater winding and the mandrel core. This would in eiiect shortcircuit a portion of the heater and overheat the remainder. It wasconsidered that these difficulties could be reduced if heaters forvacuum some way could be found to wind the heater on the mandrel so asto leave at least some open space between the mandrel and the heaterwire.

By following the method hereinafter described, applicants found itpossible to produce a mandrelsupported heater with a space between themandrel and heater wires.

It is an object of this invention to provide a mandrebsupported heaterwhich is less subject to burnouts than the conventionalmandrel-supported heaters.

It is a further object of this invention to provide a mandrel-supportedheater with a space between the mandrel and the heater.

It is a further object of this invention to pro vide a method forproducing such a mandrel-supported heater.

Still other objects and advantages of our invention will be apparentfrom the following specification.

The features of novelty which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention itself, however, both as to its fundamental principles and asto its particular embodiments will best be understood by reference tothe speciiication and accompanying drawing in which Figure 1 is a viewof a vacuum tube with a heater according to our invention, parts of thetube being broken away in the drawing for simplicity.

Figure 2 is a perspective view of an insulated mandrel at one stage ofthe practice of our in vention.

Figure 3 is greatly enlarged longitudinal section of a portion of thearrangement of Figure 2 after it has been coated according to ourinvention and the heater wire wound in position.

Figure 4 is a similar view after part of the coating has been removed inaccordance with our invention, and

Figure 5 is a similar view of the heater and mandrel after completionand Figure 6 a completed heater.

Referring now more particularly to Figure l,

t represents the mandrel supported heater as frequently used in a vacuumtube, the heater being supported within a hollow, tubular cathode itmounted in a central aperture in mica supporting discs i2 and E3. Theends of the heater wire are connected to leads it which pass out throughthe base of the tube. The electrode ssembly or mount is positioned inenvelope E5. The cathode ii is surrounded by a suitable arrangement ofgrids and an anode which are here not specifically identified since theyform no part of the present invention.

The connections from the ends of the heater wire may go directly to theleadein wires iii or the upper end of the heater wire may be connectedto the mandrel and it is used for one connection, the other being madeto the other end of the heater wire. Such construction is described inmore detail in a Wheeler application Serial Number 50,946 filedSeptember 24, 1948, and assigned to the assignee of the presentapplication.

Referring now more particularly to Figures 2 to 5, the mandrel wire itis first coated with a suitable insulating agent ll, such as aluminumoxide and sintered to fix it into a hard coat. Because such insulatedmandrels are commonly available and because in the practice of ourinvention we start with conventional insulated mandrels, the same arenot described in detail.

In accordance with our invention, we apply a special coating ila to theinsulated mandrel. This coating consists of an intimate mixture,emulsion or solution of two different substances having diiierentsolubility characteristics uniformly dispersed in a liquid carriervehicle. As an example, We may use a mixture of cellulose acetate andmethyl methacrylate (commonly called Lucite) in acetone. The mandrel iscoated with this and allowed to dry and the heater wire wound about it.This is the state of the heater shown in Figure 2. While we do not wish3 to be bound by any theory, we believe this forms a coating with fineparticles of cellulose acetate and Lucite intimately mixed in the mannershown in Figure 3 in which l8 represents finely divided particles ofcellulose acetate, and i9 similar particles of Lucite.

After the heater has been wound and tabbed with suitable connectionpieces the assembly is placed in benzene for several hours. BecauseLucite is soluble in benzene and cellulose acetate is not, a selectivedissolving action occurs removing the Lucite particles and leaving thecellulose acetate particles. Microscopic examination of the assembly atthis point shows the coating to be in the form of mounds of adherentparticles such as shown in Figure 4, with the heater wire resting on thepeaks of these mounds thereby preventing the shifting of the wire andmaintaining control of turn spacing.

The assembly is then coated with its fnal in sulation which tends tofill in the spaces left by the removal the Lucite particles. It thenfired which burns out the cellulose acetate as well as sinte the finalinsulation into a dense unitary coating. The cellulose acetate. beingslow burning, does not cause the insulation. to chip as bu ning materialwould. The burning out oi the cellulose acetate leaves voids iii wherethe acetate particles were, indicated in Figure 5, but the turns of thewire are prevented from axial shitting by the outer insulating coatingwhich extends inwardly between the wire turns into the valleys between.the mounds and to some extent, into the voids left by dissolving out theLucite. ihe layer of insulation. between wire and manorcl i'i in itsfinal state may be cl'iaracterized being spongy, that is, as an openporous body with substantial voids and cavities therein.

The result of this is a mandrel-supported heater in which the heaterturns are held in their proper space relation while at the time there issufiicient spacing between the heater turns and the mandrel to preventth develop-- ment of pressure between them due to "erences in expansionof the mandrel and heater coil. The general appearance of a completedheater is as indicated in Figure 6, one end of the heater wire beinginternally connected to mandrel wire iii and the other to tab 35 wherebyboth heater con nections may be made at the same end of the assembly.However our invention is not limited to the particular physicalarrangement shown.

While we have given an example the use of a coating of cellulose acetateand Lucite acetone, it will be clear that we are not lin.ited thereto,but may use other plastics provided they have different solubilitycharacteristics so that one may be dissolved out by selective action,leav ing the other.

While we have shown and described certain preferred embodiments of ourinvention, it will be apparent that modifications and changes may 4 bemade without departing from the spirit and scope of our invention aswill be clear to those skilled in the art.

What we claim is:

l. A new article of manufacture, comprising a mandrel-supported heaterhaving a core, a coating of insulation thereon, a heater surroundingmandrel and having substantially open space between said heater and saidcoating of insulation and a coating of refractory insulation surroundingsaid heater.

2. A new article of manufacture comprising a mandrel-supported heaterhaving a core, a coating of insulation thereon, a heater wound aroundsaid coating and spaced radially from said ooating and a coating ofinsulation surrounding said heater and projecting inwardly between theturns thereof.

3. A new article of manufacture comprising a vacuum tube having a heaterelement including an insulated mandrel, a heater wound helically aroundsaid mandrel in spaced relation thereto and a coating of refractoryinsulation surrounding said heater and projecting inwardlybetween theturns thereof.

a. A new article of manufacture comprising a mandrel-supported heaterhaving a wire core, a coating of insulation thereon, a heater coiledaround said mandrel and spaced therefrom, an outer coating of insulationsurrounding said heater, the space between said. mandrel said heatercontaining a substantial amount of open space.

A heater for electron discharge devices ineluding a refractoryinsulating core, a helical heater surrounding said core in spacedrelation with respect to said core and a coating of refractoryinsulation surrounding said heater winding and projecting inwardlybetween the turns thereof.

5. A heater for electron discharge devices including a refractoryinsulating core, a heater helically wound in a spaced away relationshipwith said core and a coating oi refractory insulation surrounding saidheater winding and pro-- iecting inwardly between the turns thereof andresting against said refractory insulating core.

2. heater electron discharge devices incluclg an inner refractory core,a spongy coating or refractory material over said core, a heater wirewound helically around said porous coating and an outer coating ofrefractory insulation surrounding said heater.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,975,870 Schradcr -l Oct. El, 1934 1,980,675 Fredenburgh Nov.13, 1934 2,131,909 Umbreit Oct. 1938 2,43%,511 Osterman et al l Jan. 13,1948 2,455,355 Combs, E. E. Dec. 7, 1948

